The obstructive sleep apnea syndrome (OSAS) is a common and serious cause of morbidity during childhood. Despite the prevalence and severity of this disease, little is understood about its pathophysiology. Normal children seldom snore, and have fewer obstructive apneas during sleep than normal adults. This suggests that the normal pediatric upper airway is more resistant to collapse than the normal adult airway, despite the fact that the pediatric airway is relatively narrower. We hypothesize that upper airway collapsibility increases with age. This age-related increase in upper airway collapsibility is caused, in part, by changes in upper airway structure, and, in part, by a decrease in upper airway neuromotor tone. Further, we hypothesize that upper airway neuromotor tone is decreased in children with OSAS compared to controls. In this proposal, the critical upper airway closing pressure (Pcrit) will be used to study the pathophysiology of childhood sleep apnea. This model evaluates pressure-flow measurements, which are objective measures of upper airway collapsibility. Specifically, (1) upper airway collapsibility will be measured across the age spectrum, from prepubertal childhood through adulthood. (2) The effect of obesity on upper airway collapsibility will be determined by comparing upper airway collapsibility between age-matched obese and non-obese children and adults. (3) The effect of neuromuscular control on upper airway collapsibility will be evaluated by testing the occlusion pressure in 100 milliseconds (P0.1) during sleep, and by determining the change in upper airway collapsibility during hypercapnia compared to room air. (4) To determine the role of upper airway reflexes in OSAS, the above studies 3 will be repeated in children with OSAS compared to matched controls. These experiments will help elucidate the underlying pathophysiology of childhood OSAS, resulting in improved management of this disease, and may ultimately help predict which children with OSAS are at risk for recurrence of disease during adulthood.